Delayed sleep phase syndrome (DSPS) is characterized by a chronic inability to fall asleep in the evening and to wake up refreshed early in the morning. These patients have an abnormally delayed sleep schedule in spite of normal sleep architecture. The goal of this study is to investigate the possibility of treating DSPS by manipulating the light-dark cycle. Patients were included in the protocol on the basis of clinical screening and polysomnography. The treatment involved a crossover study between two light-dark regimens: 1) 2 weeks of bright light (full spectrum, 2500 lux) from 7:00 a.m. to 9:00 a.m. and dark goggles from 4:00 p.m. to nightfall. and 2) 2 weeks of dim light (300 lux) from 7:00 a.m. to 9:00 a.m. and clear goggles from 4:00 p.m. to nightfall. Of nine patients treated under both conditions, the bright light-dark goggle condition has proven superior to the control treatment, as measured by self-report and morning sleep latency studies. Several of these patients have chosen to continue bright light treatment after the conclusion of the formal protocol. These findings suggest that judiciously timed bright and dark exposures are effective in advancing the circadian cycle of patients with DSPS.